1. Field of the Invention
The present invention relates generally to electromagnetic field measuring devices and more particularly to an instrument which utilizes a Luneberg lens for the measurement of the polarization of an incident electromagnetic field.
2. Description of the Prior Art
There is currently a need for an apparatus which will measure incoming electromagnetic wave fronts. Such an apparatus should have the capability of detecting radar and communications signals as well as determining the angle of arrival of any incoming other electromagnetic wave front without the scattering of the wave front which is normally associated with conventional electromagnetic field sensing apparatus. In addition, such an apparatus should have the capability of detecting the polarization of an incident electromagnetic field.
The detection and measurement of electromagnetic field radiation which may pose a biological threat to mankind has also become increasingly important in view of the rapid proliferation of electronic appliances such as microwave ovens, radar systems, medical apparatus such as diathermy equipment and the like. Both state and federal agencies have promulgated criteria for maximum electromagnetic radiation exposure in the vicinity of commercial and military radar equipment and microwave appliances. Accurate monitoring of fields emitted from such appliances and radar equipment requires that the introduction of the monitoring device into the vicinity of the microwave appliance or radar equipment will not result in significant perturbations of the electromagnetic field to be measured and a resultant loading of the radiation source. Furthermore, the relatively small permissible radiation exposure safety levels require a monitoring device that is capable of accurately measuring small field strengths.
Prior art electromagnetic field sensing devices have utilized metallic cables which tend to perturb the field being monitored thus reducing the accuracy of the resulting measurements. Prior art electromagnetic field sensing devices have also utilized a metallic antenna attached to a conducting transmission line, for example a coaxial cable, which electrically couples the antenna to a receiver unit. However, this prior art sensing device has the disadvantage that the conducting transmission is subject to line losses which limits the length of the cable to a few meters for the measurement of microwave measurements. In addition, the antenna and the transmission line of this prior art sensing device develop surface currents which are induced by the incident electromagnetic fields being sensed, thereby creating scattered electromagnetic fields. These scattered electromagnetic fields disturb the incident electromagnetic fields being sensed reducing the accuracy of the electromagnetic field measurements. Further these scattered electromagnetic fields provide a signature for enemy detectors in a hostile environment.
A more recent field measurement system employs diode detectors connected to an antenna to detect electromagnetic radiation. The diode detector employs a high-ohmic transmission line to transfer the detected signal to a readout device which allows extraneous noise to be picked up by the antenna. The bandwidth of the information transmitted on the high-ohmic transmission line is quite small, precluding the observation of short electromagnetic pulses or rapid modulation of the fields under study. Light emitting diodes connected to an antenna are impractical for the measurement of fields whose strengths even approach the small magnitude required to enforce the maximum radiation exposure criteria. The prior art light emitting diode is an active device, drawing its power from the field being monitored. Approximately 1.3 volts must be applied to the light emitting diode before any light is emitted whatsoever. However, only a few millivolts of radio-frequency or microwave energy are available from an electrically small, non-perturbing antenna in the presence of an electric field whose intensity is strong enough to just exceed the radiation exposure hazard criteria for RF or microwave appliances. In addition, the prior art light emitting diode device distorts the instantaneous signal which is received by the antenna to which it is connected, thereby reducing the accuracy of the measurement of the amplitude, phase and frequency thereof.
A need therefore exists for an electromagnetic field sensing apparatus which provides a long-sought solution to the problem of accurately measuring potentially hazardous electromagnetic radiation to confirm safe exposure criteria. There is also a need for a low scattering electromagnetic field sensing apparatus which will monitor radar and communications signals as well as measure electromagnetic fields amplitude, phase, frequency, angle of arrival and polarization on board ships, in anechoic chambers and like environments.